1. Field of the Invention
The present invention relates to a projection system, and more particularly, to a color scrollable projection system with an optical arrangement that simplifies the path of light.
2. Description of the Related Art
A projection system delivers image information to human beings. In a general projection system, in order to achieve switching for fast information processing, a light valve, such as a liquid crystal display (LCD) or a Digital Micro-mirror Device (DMD), controls the on/off operation of light emitted from a light source on a pixel-by-pixel basis and forms a picture. A magnifying projection optical system enlarges the picture to be displayed on a large screen. Projection systems are classified as 3-panel projection systems or single-panel projection systems according to the number of light valve panels that are used. Many attempts have been being made to develop simple and inexpensive single-panel projection systems that can provide a large, bright picture.
A single-panel color projection system separates white light emitted from a white light into three color beams, namely, red (R), green (G), and blue (B) beams, by using a color wheel; sequentially sends the three color beams to a single light valve; and operates the light valve according to the sequence of color beams received, thereby creating images.
A single-panel color projection system includes a simpler and smaller optical system than a three-panel projection system, in which three separate light valves form color images using an optical separation/combination system. However, the single-panel system provides only ⅓ of the light efficiency of a three-panel projection system because of the use of the color wheel.
A color scrolling method has recently been developed in which the light efficiency of a single-panel projection system is increased. In the color scrolling method, R, G, and B beams, into which white light is separated, is simultaneously sent to different locations on a light valve to form R, G, and B color bars. The R, G, and B color bars are moved at a constant speed by a color scrolling unit, and when all of the R, G, and B beams reach each pixel of the light valve, a color image is formed. If the color scrolling method is adopted, the single-panel projection system can also achieve the same light efficiency as the three-panel projection system.
FIG. 1 is a schematic diagram of a color scrolling projection system. Referring to FIG. 1, white light emitted from a lamp-type light source 102 passes through first and second lens arrays 104 and 105 and through a polarization conversion system (PCS) 106 and is focused by a lens 107. First through fourth dichroic filters 108, 110, 112, and 114 separate the light transmitted by the condenser lens 107 into R, G, and B beams which are then recombined.
More specifically, the R and B beams, for example, pass through the first dichroic filter 108 and travel along a first light path L1, while the G beam is reflected by the first dichroic filter 108 and travels along a second light path L2. The R beam and the B beam on the first light path L1 are separated by the second dichroic filter 110. The R beam continues along the first light path L1, passing through the second dichroic filter 110, and the second dichroic filter 110 reflects the B beam along a third light path L3.
The G and B beams, which travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected, respectively, by the third dichroic filter 112, and are combined. The R, G, and B beams are then all combined by the fourth dichroic filter 114. The combined beam passes through a polarization beam splitter (PBS) 128 and is incident on a light valve 130. Reference numeral 126 indicates a polarizer, reference numeral 132 indicates an analyzer, reference numeral 121 indicates a reflection filter which reflects an R beam, and reference numeral 122 indicates a reflection filter which reflects a B beam.
First, second, and third prisms 120, 116 and 118 are disposed in the first through third light paths L1, L2, and L3, respectively. As the first, second, and third prisms 120, 116, and 118 rotate at a uniform speed, R, G, and B color bars formed on the light valve 130 are properly scrolled.
First, second, and third slits 119, 115, and 117, for determining the widths of the R, G, and B beams, are installed in the first, second, and third light paths L1, L2, and L3, respectively, and in front of the first, second, and third prisms 120, 116, and 118, respectively. The widths of color bars formed on the light valve 130 depend on the widths of the first, second, and third slits 119, 115, and 117. Therefore, by varying the widths of the first, second, and third slits 119, 115, and 117, the widths of the color bars may be decreased, and thus, black bars K may be formed between adjacent color bars. Alternately, the R, G, and B bars may be enlarged such that overlapping portions P may be formed between adjacent color bars.
R, G, and B beams transmitted by the first, second, and third slits 119, 115, and 117 are scrolled by rotation of the first, second, and third prisms 120, 116, and 118, which serve as a scrolling unit.
In the above-described conventional projection system, while the first through fourth dichroic filters 108, 110, 112, and 114 separate light into R, G, and B beams and combining the R, G, and B beams, R, G, and B color bars having desired beam widths can be scrolled on the light valve 130 by using the first through third slits 119, 115, and 117 and the first through third rotating prisms 120, 116, and 118, which are disposed on first through third light paths L1, L2, and L3.
The scrolling of the R, G, and B color bars due to the rotation of the first through third prisms 120, 116, and 118 is illustrated in FIG. 2. Scrolling represents the movement of color bars formed on the light valve 130 when the first, second, and third prisms 120, 116, and 118 corresponding to R, G, and B colors, respectively, are synchronously rotated.
A color image is obtained by controlling the light valve 130 in synchronization with the movement of the color bars formed on the light valve 130. In other words, beams incident on the light valve 130 via the PBS 128 form a color image as individual pixels of the light valve 130 are turned on or off according to an image signal. The color image is magnified by a projection lens 134 and projected onto a screen (not shown).
Due to the use of the complex system described above and the complex light paths utilized therein, the conventional projection system is bulky and its assembly is complicated. In particular the above-described system requires a plurality of dichroic filters and both a slit and a scrolling unit for each of the R, G, and B beams. Hence, the conventional projection system is complicated, and requires a large number of optical components.
Furthermore, since color scrolling is performed by individually rotating each of the three prisms 120, 116, and 118, synchronization of the prisms with the light valve 130 is difficult. In other words, in order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be scrolled at a constant speed. Hence, the conventional projection system must synchronize the light valve 130 with the three prisms 120, 116, and 118 in order to achieve proper scrolling. However, controlling the synchronization is not easy. Due to the circular motion of the scrolling prisms 120, 116, and 118, the color scrolling speed is irregular, consequently deteriorating the quality of the resultant image.
Three motors for rotating the first, second, and third scrolling prisms 120, 116, and 118 generate a lot of noise during operation. Further, the cost of manufacturing a system with three motors is higher than that of manufacturing a color wheel type projection system which utilizes a single motor.